1. Field of the Invention
This invention relates generally to the manufacture of data recording media and particularly to the manufacturing of durable recording medium surfaces for high performance disk drives.
2. Description of the Related Art
Magnetic recording media on disk drives typically consist of thin film magnetic alloys deposited on a disk. In hard disk technology, the disk is usually an aluminum disk with a surface prepared to receive the magnetic alloy thin film. Techniques for depositing the thin film of magnetic alloy include electrolytic or electroless plating from aqueous solutions, or more commonly in current technology glow-discharge sputter-deposition processes. To improve the durability of the thin film magnetic alloy, existing technologies include depositing a thin film of hard overcoat material over the alloy, either directly on the magnetic alloy, or with intervening layers used for adhesion or for other purposes. Typically, the overcoat material in recently manufactured disk drives consists of amorphous carbon or hydrogenated carbon. The overcoat provides a hard, wear resistant layer that protects the disk from contact with the read/write head during start and stop operations, and in other transient contact situations.
As consumers demand software with ever increasing features and capabilities, the need for additional disk storage becomes more pressing. Space and weight limitations on portable and desktop computers require disk drive manufacturers to rely on higher data densities as one way to address this need. However, higher data densities impose new challenges on the disk drive maker.
As the data density increases on disk drives, the flying height for the read/write heads used with the disk is becoming lower and lower, and the bit cell size is becoming smaller and smaller. The lower flying heights tend to cause greater wear on the disk surface, and reduce the reliability and useful life of the disk. The small bit cell sizes increase demands on the magnetic performance of the recording medium.
To provide more durable disks to accommodate the lower flying heights, nitrogenated carbon overcoats have been developed. See, for example, U.S. Pat. No. 5,453,168, by Nelson et al., entitled Method for Forming Protective Overcoatings for Metallic-Film Magnetic-Recording Mediums, which discusses a problem of adhesion of wear resistant protective overcoats to magnetic alloys, and provides a good discussion of the deposition technologies used in the magnetic recording disk industry. Nelson, et al. characterize their invention as two layers of sputtered material on cobalt-alloy metallic-film magnetic disks, in which the first layer is an interfacial adhesion layer of sputtered titanium or related material, and the second layer is preferably sputtered titanium diboride or sputtered amorphous nitrided carbon.
Published studies concerning the properties of amorphous carbon nitride coatings, include the Li et al., "Composition, Structure and Tribological Properties of Amorphous Carbon Nitride Coatings", Surface and Coatings Technology, 68/69 (1994), pages 611-615; Li et al., "Nano-Indentation Studies of Ultra High Strength Carbon Nitride Thin-Films", J. Appl. Phys. 74 (1), 1 Jul. 1, 1993, pages 219-223; and Li et al., "Infra-Red Absorption and Nuclear Magnetic Resonance Studies of Carbon Nitride Thin Films Prepared by Reactive Magnetron Sputtering", J. Vac. Sci. Technol. A 12 (4), July/August, 1994, pages 1470-1473.
Thus nitrogenated carbon has been reported to produce films that perform very well in terms of hardness, smoothness, and other properties. On the other hand, the presence of nitrogen in the sputter chamber during deposition of the magnetic alloy, has been discovered to cause reduction in coercivity (See, Kahn, et al., "Effects of Nitrogen, Oxygen and Air on the Magnetic Properties of Sputtered CoCrTa Thin Film Recording Discs", IEEE Transactions on Magnetics, Vol. 24, No. 6, November 1988). Even if nitrogen is introduced only into the carbon deposition chambers used for depositing the overcoat, while injecting pure argon or other optimized gas into the deposition chamber for the magnetic alloy, a reduction in coercivity has been observed. The reduction in magnetic coercivity may be due to contamination of the deposition chamber used for the magnetic alloy by excess nitrogen which is carried over from the carbon deposition chamber or chambers, or contamination of the magnetic alloy due to chemical or physical migration from the nitrogenated carbon layer. Alternatively, stresses between the nitrogenated carbon film and the underlying magnetic film may be responsible for the effect.
The reduction in coercivity caused by nitrogen causes increased susceptibility to "self-demagnetization" effects, particularly in high density disks. In high density disks, small bit cell sizes are required. However, the small bit cell size results in transitions in magnetic polarity which are very close together. These close together transitions tend to cause "self-demagnetization". To counteract self-demagnetization, higher coercivity is required for the magnetic media to be used. Thus, if nitrogenated carbon is to be successfully used in high density disk drives, then a solution to the problem of reduced coercivity caused by the nitrogen is required for high density applications.
In summary, nitrogenated carbon overcoats offer some tantalizing properties to designers of high density disk drives. On the other hand, nitrogen has been demonstrated to have deleterious effects on magnetic films. If these negative effects are ignored, the manufacturer risks lower yield, especially in high density products. Lower yields in turn result in higher per disk manufacturing costs.
Thus, it is desirable to increase the durability of the magnetic recording medium on disks, without reducing the magnetic performance, to accommodate both greater data density and longer life for disk drives.